diff --git a/src/index.js b/src/index.js
index 60221e6..3eaece8 100644
--- a/src/index.js
+++ b/src/index.js
@@ -39,6 +39,56 @@ const server = http.createServer((req, res) => {
     return;
   }
 
+  // Subtask routes: /tasks/:id/subtasks
+  const subtaskMatch = url.pathname.match(/^\/tasks\/(\d+)\/subtasks$/);
+  if (subtaskMatch) {
+    const parentId = parseInt(subtaskMatch[1], 10);
+
+    if (method === 'GET') {
+      const subtasks = store.listSubtasks(parentId);
+      if (subtasks === null) {
+        res.writeHead(404);
+        res.end(JSON.stringify({ error: 'parent task not found' }));
+        return;
+      }
+      res.writeHead(200);
+      res.end(JSON.stringify(subtasks));
+      return;
+    }
+
+    if (method === 'POST') {
+      let body = '';
+      req.on('data', chunk => { body += chunk; });
+      req.on('end', () => {
+        try {
+          const { title, description } = JSON.parse(body);
+          if (!title) {
+            res.writeHead(400);
+            res.end(JSON.stringify({ error: 'title is required' }));
+            return;
+          }
+          const result = store.createSubtask(parentId, title, description);
+          if (result.error === 'parent not found') {
+            res.writeHead(404);
+            res.end(JSON.stringify({ error: 'parent task not found' }));
+            return;
+          }
+          if (result.error === 'subtasks of subtasks are not allowed') {
+            res.writeHead(400);
+            res.end(JSON.stringify({ error: 'subtasks of subtasks are not allowed' }));
+            return;
+          }
+          res.writeHead(201);
+          res.end(JSON.stringify(result));
+        } catch (e) {
+          res.writeHead(400);
+          res.end(JSON.stringify({ error: 'invalid JSON' }));
+        }
+      });
+      return;
+    }
+  }
+
   const taskMatch = url.pathname.match(/^\/tasks\/(\d+)$/);
   if (taskMatch) {
     const id = parseInt(taskMatch[1], 10);
diff --git a/src/store.js b/src/store.js
index fe8b2b0..b6252de 100644
--- a/src/store.js
+++ b/src/store.js
@@ -10,6 +10,24 @@ class TaskStore {
       title,
       description,
       completed: false,
+      parentId: null,
+      createdAt: new Date().toISOString(),
+    };
+    this.tasks.set(task.id, task);
+    return task;
+  }
+
+  createSubtask(parentId, title, description = '') {
+    const parent = this.tasks.get(parentId);
+    if (!parent) return { error: 'parent not found' };
+    if (parent.parentId !== null) return { error: 'subtasks of subtasks are not allowed' };
+
+    const task = {
+      id: this.nextId++,
+      title,
+      description,
+      completed: false,
+      parentId,
       createdAt: new Date().toISOString(),
     };
     this.tasks.set(task.id, task);
@@ -24,6 +42,12 @@ class TaskStore {
     return Array.from(this.tasks.values());
   }
 
+  listSubtasks(parentId) {
+    const parent = this.tasks.get(parentId);
+    if (!parent) return null;
+    return Array.from(this.tasks.values()).filter(t => t.parentId === parentId);
+  }
+
   update(id, updates) {
     const task = this.tasks.get(id);
     if (!task) return null;
@@ -36,6 +60,15 @@ class TaskStore {
   }
 
   delete(id) {
+    if (!this.tasks.has(id)) return false;
+
+    // Cascade delete: remove all subtasks of this task
+    for (const [taskId, task] of this.tasks) {
+      if (task.parentId === id) {
+        this.tasks.delete(taskId);
+      }
+    }
+
     return this.tasks.delete(id);
   }
 }
diff --git a/src/store.test.js b/src/store.test.js
index d3c335c..5bc8443 100644
--- a/src/store.test.js
+++ b/src/store.test.js
@@ -46,4 +46,93 @@ describe('TaskStore', () => {
     assert.strictEqual(store.delete(1), true);
     assert.strictEqual(store.get(1), null);
   });
+
+  it('creates a task with parentId null by default', () => {
+    const store = new TaskStore();
+    const task = store.create('Top-level task');
+    assert.strictEqual(task.parentId, null);
+  });
+});
+
+describe('TaskStore - Subtasks', () => {
+  it('creates a subtask under a valid parent', () => {
+    const store = new TaskStore();
+    const parent = store.create('Parent task');
+    const subtask = store.createSubtask(parent.id, 'Child task', 'Details');
+    assert.strictEqual(subtask.parentId, parent.id);
+    assert.strictEqual(subtask.title, 'Child task');
+    assert.strictEqual(subtask.description, 'Details');
+    assert.strictEqual(subtask.completed, false);
+    assert.ok(subtask.id > parent.id);
+  });
+
+  it('returns error when creating subtask under non-existent parent', () => {
+    const store = new TaskStore();
+    const result = store.createSubtask(999, 'Orphan task');
+    assert.deepStrictEqual(result, { error: 'parent not found' });
+  });
+
+  it('rejects creating a subtask of a subtask (single-level nesting)', () => {
+    const store = new TaskStore();
+    const parent = store.create('Parent');
+    const subtask = store.createSubtask(parent.id, 'Child');
+    const result = store.createSubtask(subtask.id, 'Grandchild');
+    assert.deepStrictEqual(result, { error: 'subtasks of subtasks are not allowed' });
+  });
+
+  it('lists subtasks of a parent', () => {
+    const store = new TaskStore();
+    const parent = store.create('Parent');
+    store.createSubtask(parent.id, 'Child 1');
+    store.createSubtask(parent.id, 'Child 2');
+    const subtasks = store.listSubtasks(parent.id);
+    assert.strictEqual(subtasks.length, 2);
+    assert.strictEqual(subtasks[0].title, 'Child 1');
+    assert.strictEqual(subtasks[1].title, 'Child 2');
+  });
+
+  it('returns null when listing subtasks of non-existent parent', () => {
+    const store = new TaskStore();
+    assert.strictEqual(store.listSubtasks(999), null);
+  });
+
+  it('returns empty array when parent has no subtasks', () => {
+    const store = new TaskStore();
+    const parent = store.create('Parent');
+    const subtasks = store.listSubtasks(parent.id);
+    assert.deepStrictEqual(subtasks, []);
+  });
+
+  it('cascade deletes subtasks when parent is deleted', () => {
+    const store = new TaskStore();
+    const parent = store.create('Parent');
+    const child1 = store.createSubtask(parent.id, 'Child 1');
+    const child2 = store.createSubtask(parent.id, 'Child 2');
+    store.delete(parent.id);
+    assert.strictEqual(store.get(parent.id), null);
+    assert.strictEqual(store.get(child1.id), null);
+    assert.strictEqual(store.get(child2.id), null);
+  });
+
+  it('GET /tasks returns all tasks including subtasks (backward compat)', () => {
+    const store = new TaskStore();
+    store.create('Parent');
+    store.createSubtask(1, 'Child');
+    const all = store.list();
+    assert.strictEqual(all.length, 2);
+  });
+
+  it('only returns subtasks belonging to the specified parent', () => {
+    const store = new TaskStore();
+    const parent1 = store.create('Parent 1');
+    const parent2 = store.create('Parent 2');
+    store.createSubtask(parent1.id, 'Child of P1');
+    store.createSubtask(parent2.id, 'Child of P2');
+    const subtasks1 = store.listSubtasks(parent1.id);
+    assert.strictEqual(subtasks1.length, 1);
+    assert.strictEqual(subtasks1[0].title, 'Child of P1');
+    const subtasks2 = store.listSubtasks(parent2.id);
+    assert.strictEqual(subtasks2.length, 1);
+    assert.strictEqual(subtasks2[0].title, 'Child of P2');
+  });
 });